Fluorescent lamps use phosphors excited by electrons to create light. Fluorescent lamps ionize mercury vapor in a glass tube which causes electrons in the gas to emit photons at ultraviolet (UV) frequencies. The UV light is converted into visible light using a phosphor coating on the inside of the glass tube. The most common fluorescent lamp is a hot cathode lamp. A hot cathode lamp includes a glass tube filled with an inert gas (e.g, argon) at low pressure. An electrode is located on each side of the glass tube. A ballast regulates alternating current (AC) power that is applied to the electrodes. In certain applications where light from fluorescent lamps may be required even during power outages, power supplies with battery back-up have been implemented to power the lamps during power outages. Nickel Cadmium (NiCad) batteries are typically used in fluorescent light back-up power supplies.